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    <title>Document</title>
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    <script>
        //预解析案例
        //案例1
        function cook() {
            console.log(age);
        }
        cook();
        var age = 10; //位置不同输出不同

        // 案例2
        var num = 10;
        function fn(){
            console.log(num);
            var num = 20;
            console.log(num);
        }
        fn();
      // 案例3
        // var a = 18;
        // f1();

        // function f1() {
        //     var b = 9;
        //     console.log(a);
        //     console.log(b);
        //     var a = '123';
        // }

    </script>
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